This invention relates to an air conditioning system for automotive vehicles, and more particularly to a system of this kind which is provided with a variable capacity compressor in which the delivery quantity or capacity is varied by utilizing pressure in a high pressure zone.
Air conditioning systems are known, e.g. from Japanese Provisional Patent Publications (Kokai) Nos. 58-158382 and 62-129593, in which variable capacity compressors are employed, the delivery quantity or capacity of which is varied by utilizing a pressure in a high pressure zone in the compressor. Such air conditioning systems have the following disadvantages which are encountered when the pressure in the high pressure zone is low, that is, for example, when the ambient air temperature is extremely low, or when the compressor starts to operate when the ambient air temperature is extremely low.
According to the Japanese Provisional Patent Publication No. 58-158382, a capacity control means is provided, which selectively communicates the interior of a crankcase with a suction cavity or a discharge cavity in response to both suction pressure and discharge pressure so that when the difference between the suction pressure and the discharge pressure reaches a predetermined value, the difference between pressure within the the crankcase and the suction pressure becomes zero to obtain the maximum delivery quantity, while the delivery quantity is reduced as the difference between the suction pressure and the discharge pressure is increased to cause an increase in the difference between the pressure within the crankcase and the suction pressure, that is, the delivery quantity is reduced as the discharge pressure and suction pressure increase.
In this known compressor, when the discharge pressure is low, even if the capacity control means is operated to introduce the discharge pressure into the crankcase, the pressure within the crankcase is not increased to a sufficient level, which causes an insufficient increase in the difference between the pressure within the crankcase and the suction pressure, thereby failing to obtain a sufficient reduction in the delivery quantity of the compressor.
On the other hand, according to the Japanese Provisional Patent Publication No. 62-129593, the compressor comprises a control element which is circumferentially movable in response to the difference between pressure within a first chamber supplied with low or suction pressure and pressure within a second chamber supplied with high or discharge pressure, the circumferential position of which determines the compression commencing timing of the compressor, a communication passage communicating the second chamber with a low pressure side, and a control valve means arranged across the communication passage for closing same when the suction pressure is higher than a predetermined value, and opening same when the suction pressure is lower than the predetermined value, wherein the control element is circumferentially moved in response to a change in the pressure difference between the first and second chambers to vary the compression commencing timing and hence vary the delivery quantity. However, according to this known compressor, when the ambient air temperature is extremely low and accordingly the discharge pressure is low, even if the control valve means is operated at and immediately after the start of the compressor, the pressure within the first chamber does not increase to such a sufficient level as to enable proper control of the delivery quantity of the compressor.